1. Field of the Invention
The present invention relates to a real-image finder optical system applied to a camera in which the finder optical system is provided independently from the photographing optical system.
2. Description of the Related Art
A real-image finder optical system is constituted by a positive objective optical system for forming an inverted object image which is upside down and reversed from left to right, an erecting optical system for erecting the inverted object image formed by the objective optical system to the proper orientation, and an eyepiece lens element (eyepiece optical system) for viewing the erected object image reinverted by the erecting optical system. In such a real-image finder optical system, a focal point of the objective optical system is arranged to coincide with a focal point of the eyepiece lens element (eyepiece optical system), on the same optical axis. The overall length of the optical system therefor becomes longer. Accordingly, the thickness (length from front to back) of a camera, which is necessary to incorporate such an optical system, is undesirably increased. In a conventional real-image finder optical system, an arrangement has been made to shorten the distance between an object image and the erecting optical system by deflecting the optical axis through a plurality of reflection surfaces provided between the object image and the eyepiece optical system, thereby the front-to-back length of the finder optical system has been made shorter.
As an example for miniaturizing the real-image finer optical system in the front-to-back direction, a U.S. Pat. No. 5,235,460 has taught that reflection surfaces are provided along an object-side finder optical path with respect to the position where an object image is formed (hereinafter, an object image). An optical axis which is reflected by a reflection surface is deflected at a right-angle with respect to an optical axis which is incident thereon. According to this arrangement, the overall length of the finder optical system in the front-to-back direction can be reduced, thereby it is possible to reduce the thickness (length from front to back) of a camera, which is necessary to incorporate such an optical system. Furthermore, according to this arrangement, out of the reflection surfaces required to constitute the erecting optical system, the number of reflection surfaces between the object image and the eyepiece lens element (eyepiece optical system) can be reduced, thereby the optical path between the object image and the eyepiece lens element can be shortened. Consequently, the focal length of the eyepiece lens element can be comparatively shortened, and a real-image finder optical system having a high finder magnification can be obtained.
However, if a further attempt is made to reduce the length of the real-image finder optical system of this type in the front-to-back direction, there is a need to shorten the focal length of the objective optical system, since the objective optical system occupies the majority of the overall length of the finder optical system. On the other hand, the length of the eye relief, which is the degree of ease for viewing through the eyepiece lens element (eyepiece optical system), is proportional to the focal length of eyepiece lens element (eyepiece optical system). As a result, there is a limit to shorten the focal length of the eyepiece lens element (eyepiece optical system) while an acceptable eye relief is maintained.
Accordingly, if an attempt is made to further miniaturize a conventional real-image finder optical system, the focal lengths of both objective optical system and eyepiece lens element (eyepiece optical system) have to be reduced. Consequently, the eye relief becomes shorter, so that ease for viewing may not properly be secured. Still further, the focal length of the objective optical system becomes excessively short with respect to the focal length of the eyepiece lens element (eyepiece optical system), thereby the finder magnification (fo/fe; fo: the focal length of the objective optical system; fe: the focal length of the eyepiece lens element (eyepiece optical system)) decreases, and it becomes difficult for an operator to view an object image, and the quality of the object image may deteriorate.
In addition, in order to superimpose finder-field information, such as the field frame, the distance-measuring frame and so forth, onto the object image, a display member which is constituted by a transparent member having the finder field information formed thereon is provided in the vicinity of the object image formed by the objective optical system. However, in a conventional real-image finder optical system, dust and scratches on the surface of the transparent member would also appear with the object image. Particularly, in the case where the focal length of the eyepiece optical system is set to be short, such dust and scratches are enlarged, which may obstruct the field of view.